OkHttpClient client = new OkHttpClient().newBuilder()
  .build();
MediaType mediaType = MediaType.parse("application/json");
RequestBody body = RequestBody.create(mediaType, "{\r\n  \"inputs\": {\r\n    \"calculationMethod\": \"Design\",\r\n    \"selectedFluidSide1\": {\r\n      \"fluidId\": 53,\r\n      \"isGas\": false,\r\n      \"inputValue\": null\r\n    },\r\n    \"selectedFluidSide2\": {\r\n      \"fluidId\": 53,\r\n      \"isGas\": false,\r\n      \"inputValue\": null\r\n    },\r\n    \"coCurrent\": false,\r\n    \"selectedHeatExchangers\": [\r\n      {\r\n        \"heatExchangerInputType\": \"Parent\",\r\n        \"id\": 1\r\n      }\r\n    ],\r\n    \"heatLoad\": {\r\n      \"amount\": 1500,\r\n      \"unit\": \"HeatLoadUnit.Kilowatt\"\r\n    },\r\n    \"inletTemperatureSide1\": {\r\n      \"amount\": 80,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"inletTemperatureSide2\": {\r\n      \"amount\": 20,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"outletTemperatureSide1\": {\r\n      \"amount\": 40,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"outletTemperatureSide2\": {\r\n      \"amount\": 50,\r\n      \"unit\": \"TemperatureUnit.DegreeCelsius\"\r\n    },\r\n    \"flowSide1\": null,\r\n    \"flowSide2\": null,\r\n    \"maxPressureDropSide1\": {\r\n      \"amount\": 20000,\r\n      \"unit\": \"PressureUnit.Pascal\"\r\n    },\r\n    \"maxPressureDropSide2\": {\r\n      \"amount\": 20000,\r\n      \"unit\": \"PressureUnit.Pascal\"\r\n    },\r\n    \"numberOfPasses\": null,\r\n    \"numberOfPlates\": null,\r\n    \"numberOfPlatesStack1\": null,\r\n    \"numberOfPlatesStack2\": null,\r\n    \"oversurface\": null,\r\n    \"portSwitch\": true,\r\n    \"autoperformance\": false\r\n  }\r\n}");
Request request = new Request.Builder()
  .url("https://api.swep.net/calculateSinglePhase")
  .method("POST", body)
  .addHeader("X-API-KEY", "YOUR_API_KEY")
  .addHeader("Content-Type", "application/json")
  .build();
Response response = client.newCall(request).execute();